Fine grained cemented carbide with refined structure

ABSTRACT

The present invention relates to a fine grained WC-Co cemented carbide. By adding an extremely small amount of Ti, V, Zr, Ta or Nb alone or in combinations, a grain refined cemented carbide structure with less abnormal WC-grains has been obtained.

BACKGROUND OF THE INVENTION

The present invention relates to a fine grained cemented carbidecontaining chromium. By adding small but well controlled amounts of Ti,Nb, V, Zr or Ta alone or in combinations, a further grain refinedcemented carbide structure has been obtained without embrittlingsecondary phases.

Cemented carbide cutting tools with a grain refined structure are usedtoday to a great extent for the machining of steel, hardened steel,stainless steels and heat resistant alloys in applications with highdemands on both toughness and wear resistance. Another importantapplication is in microdrills for the machining of printed circuitboard, so-called PCB-drills. In these types of applications it is knownthat the amount and size of defects, such as abnormal WC-grains, smallprecipitates of embrittling phases, porosity, binder phase lakes, arecritical for the mechanical properties, e.g. fracture toughness andtransverse rupture strength (TRS).

Common grain growth inhibitors include vanadium, chromium, niobium,tantalum or compounds involving these. When added, generally as carbidesand in an amount of from about 0.15 to about 1.5 wt-%, they limit graingrowth during sintering, but they also have undesirable side effects,affecting the toughness behaviour in an unfavourable direction. Vanadiumis known to be the most effective grain growth inhibitor but is alsoparticularly detrimental due to its negative influence on the toughnessproperties. Normal levels of vanadium additions are also known to resultin precipitation of embrittling phases in the WC/Co grain boundaries.

In order to achieve a fine, as sintered grain size of less than about0.6 μm for cemented carbides with Cr/Co-ratios of from about 0.04 toabout 0.06, very fine WC raw material is needed and the sintering isgenerally performed at a relatively low temperature, e.g. close to atemperature of about 1360° C. and often by sinterHIP or followed by hotisostatic pressing in order to obtain a dense structure. Such aproduction route of course increases the production cost, but alsoincreases the risk of uneven binder phase distribution. In addition, theuse of these very fine WC-raw materials gives powders with inferiorcompaction properties, resulting in powders that are difficult tocompact to complicated geometries such as cutting inserts.

According to WO 99/13120, the amount of grain growth inhibitors can bereduced if a carbon content of the cemented carbide close to eta-phaseformation is chosen.

JP-A-11-152535 discloses a process to manufacture fine grained tungstencarbonitride-cobalt hard alloys using tungsten carbonitride as a rawmaterial. JP-A-10-324942 and JP-A-10-324943 disclose methods to producefine grained cemented carbide by adding the grain growth inhibitors asnitrides. In order to avoid pore formation by denitrification of thenitrides sintering is performed in a nitrogen atmosphere.

EP-A-1500713 discloses a method of making a fine grained tungstencarbide-cobalt cemented carbide comprising mixing, milling according tostandard practice followed by sintering. By introducing nitrogen at apressure of more than 0.5 atm into the sintering atmosphere afterdewaxing but before pore closure a grain refinement including reducedgrain size and less abnormal grains can be obtained.

EP-A-1803830 discloses cemented carbides which include WC having anaverage particle diameter of <0.3 μm as a hard phase and 5.5 to 15 wt-%of at least one iron group metal element as a binder phase, andcomprise, in addition to the above hard phase and binder phase, 0.005 to0.06 wt-% of Ti, Cr in a weight ratio relative to the binder phase of0.04 to 0.2. Especially, the above cemented carbides contain no Ta.

According to US 2006/029511, there is provided a method of making a finegrained tungsten carbide-cobalt cemented carbide comprising mixing,milling according to standard practice followed by sintering. Byintroducing nitrogen at a pressure of more than 0.5 atm into thesintering atmosphere after dewaxing but before pore closure a grainrefinement including reduced grain size and less abnormal grains can beobtained.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to avoid or alleviate theproblems of the prior art.

It is further an object of the present invention to provide fine orsubmicron cemented carbide cutting tools with improved binder phasedistribution and reduced amounts of abnormal WC grains giving cementedcarbide tools with improved mechanical and cutting properties.

In one aspect of the invention, there is provided a WC-Co cementedcarbide comprising a composition of from about 3 to about 15 wt-% Co andCr such that the Cr/Co-ratio by weight is from about 0.05 to about 0.15and balance WC and in addition ppm levels of

-   -   one of the following additional elements V, Nb, Zr, Ta then with        a grain size of WC of from about 0.1 to about 2.0 μm or    -   mixtures of at least two of Ti, V, Nb, Zr, Ta, then with a grain        size of WC of more than about 0.3 up to about 2.0 μm,    -   the ratio of Me/Co=(at-% Ti+at-% V+at-% Nb+at-% Zr+at-% Ta)/at-%        Co is lower than or equal to about 0.014-(CW_Cr)*0.008 and        higher than about 0.0005, with a CW_Cr ratio of from about 0.79        to about 0.95, with CW_Cr=(magnetic-% Co+1.13*wt % Cr)/wt % Co        where magnetic-% Co is the weight percentage of magnetic Co and        wt-% Co is the weight percentage of Co in the cemented carbide.

In another aspect of the invention, there is provided a method of makinga WC-Co cemented carbide as described in the preceding paragraphcomprising mixing and wetmilling of powders of WC and Co, pressing andsintering wherein ppm levels of Ti, V, Nb, Zr or Ta or mixtures thereof,except Ti alone, as pure metals or as a carbides, nitrides and/orcarbonitrides or mixtures thereof are added to the powder mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a back scattered SEM micrograph of the microstructure of acemented carbide according to prior art.

FIG. 2 shows a back scattered SEM micrograph of the microstructure of acemented carbide according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has now surprisingly been found that a pronounced grain refiningeffect in combination with an improved binder phase distribution can beobtained by the introduction of one of V, Nb, Zr, Ta or mixtures of atleast two of Ti, V, Nb, Zr, Ta, on ppm-level in a chromium containingcemented carbide. The present invention provides a material that can besintered at about 1360° C. or higher maintaining the fine grain size andbinder phase distribution despite that the higher Cr/Co-ratio decreasesthe melting temperature of the binder and, thus, gives a longer time inthe liquid state during the sintering where grain growth is rapid. Thepresent invention allows for the same grain size to be achieved alsoallowing for higher carbon balances applicable for CVD coating.

The cemented carbide body comprises tungsten carbide with from about 3to about 15 wt-% Co and Cr such that the Cr/Co-ratio by weight is fromabout 0.05 to about 0.15 and balance WC. In addition to that ppm levelsof:

-   -   one of the following additional elements V, Nb, Zr, Ta then with        a grain size of WC of from about 0.2 to about 2.0 μm, or    -   mixtures of at least two of Ti, V, Nb, Zr, Ta, then with a grain        size of WC of more than about 0.3 up to about 2.0 μm. The ratio        of        Me/Co=(at-% Ti+at-% V+at-% Nb+at-% Zr+at-% Ta)/at-% Co        is lower than or equal to about 0.014-(CW_Cr)*0.008 and higher        than about 0.0005, preferably higher than about 0.0007, with        CW_Cr from about 0.79 to about 0.95, preferably from about 0.80        to about 0.92, where        CW_Cr=(magnetic-% Co+1.13*wt % Cr)/wt-% Co        where magnetic-% Co is the weight percentage of magnetic Co and        wt-% Co is the weight percentage of Co in the cemented carbide.        The CW_Cr is a function of the W content in the Co binder phase.        A CW_Cr of about 1 corresponds to a very low W-content in the        binder phase and a CW_Cr of 0.75-0.8 corresponds to a high        W-content in the binder phase.

The sintered body may also contain small amounts of precipitations offurther additional phase or phases such as eta-phase, MX or M₇X₃, M₃X₂where M=(V+Zr+Ti+Ta+Nb+Co+Cr+W) and X=C or N may be allowed to a volumefraction of maximum 0.5 vol % without detrimental effects.

In one embodiment, the further additional element is Zr alone or incombination with one or more of the aforesaid additional elements.

In another embodiment, the further additional elements are Zr and Ti.

In yet another embodiment, the further additional elements are Ta andTi.

In another embodiment, the cemented carbide contains from about 0.01 toabout 0.10 wt-% N.

In yet another embodiment, the further additional element(s) is amixture of Ti and Ta, having a N-content of more than about 0.02 wt-%,preferably more than about 0.03 wt-% but less than about 0.1 wt-%.

The method of making the cemented carbide according to the presentinvention with a composition according to above comprises mixing,milling, pressing and sintering of submicron tungsten carbide-cobaltbodies according to standard practice. Ppm levels of Ti, V, Nb, Zr or Taor mixtures thereof, except Ti alone, are added as pure metals or ascarbides, nitrides and/or carbonitrides or mixtures thereof in suchamounts that the Me/Co and CW values according to above are fulfilled inthe sintered cemented carbide.

The invention is additionally illustrated in connection with thefollowing examples, which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the examples.

EXAMPLE 1

Cemented carbide mixtures 1A-1K with the compositions in wt-% accordingto table 1a and WC as rest were prepared. The WC-powder had an FSSSgrain size of 0.8 μm as supplied from H.C. Starck and fine grain (0.5μm) cobalt from Umicore and H.C. Starck fine grain Cr₃C₂, TiC, VC, ZrC,TaC and NbC:

TABLE 1a Variant Co Cr Ti V Zr Ta Nb 1A 7 0.8 — — — — — comp 1B 7 0.80.023 — 0.01 — — inv 1C 7 0.8 0.032 — 0.005 — — inv 1D 7 0.8 — 0.0240.01 0.014 — inv 1E 7 0.8 — 0.034 0.005 — — inv 1F 7 1.0 0.023 0.0120.005 — — inv 1G 7 0.7 0.014 — — 0.010 — inv 1H 7 0.9 — — 0.020 0.01 —inv 1I 9.25 1.2 — — — 0.013 0.0072 inv 1J 9 0.9 0.023 — — 0.014 — inv 1K9 0.36 — — — — — ref

The mixtures were wet milled, dried, compacted and sintered to bodies at1360 and 1410° C., respectively, in a protective atmosphere of Ar at apressure of 40 mbar for 1 h. After sintering CW_Cr and0.014-(CW_Cr)*0.008 for the two sintering temperatures were determinedand reported in table 1b together with the Me/Co.

TABLE 1b Sintering temp. ° C. 1410 1360 1410 0.014 − 1360 0.014 − Me/CoCW_Cr (CW_Cr) * 0.008 CW_Cr (CW_Cr) * 0.008 1A NA 0.94 NA 0.90 NA 1B0.00491 0.96 0.00633 0.92 0.00663 1C 0.00609 0.94 0.00649 0.91 0.006711D 0.00546 0.90 0.00681 0.94 0.00647 1E 0.00600 0.89 0.00689 0.930.00655 1F 0.00645 0.88 0.00697 0.86 0.00709 1G 0.00258 0.85 0.007180.83 0.00736 1H 0.00228 0.76 0.00790 0.77 0.00786 1I 0.00094 0.950.00643 0.88 0.00697 1J 0.00228 0.85 0.00718 0.85 0.00718 1K NA 0.90 NA0.87 NA NA = not applicable

In table 1c the as sintered grain size is given for the sinteringtemperatures 1410 and 1360° C. The sintered grain size was determinedfrom hardness (Hv30) and coercivity measurements.

TABLE 1c Variant 1A 1B 1C 1D 1E 1F 1G 1H 1I 1J 1K Sintered at 1410° C.Dwc, μm 0.90 0.77 0.74 0.82 0.82 0.77 0.80 0.82 0.61 0.70 0.96 Sinteredat 1360° C. Dwc, μm 0.82 0.70 0.70 0.79 0.78 0.75 0.76 0.77 0.59 0.640.88

The surprising effect of low additions cubic carbide formers is clearlydemonstrated at two different sintering temperatures.

EXAMPLE 2

Cemented carbide mixtures 2A, 2B and 2C with compositions given in Table2a were prepared. The WC-powder had an FSSS grain size of 0.9 μm with aN-content of 0.04 wt-% and fine grain (0.5 μm) cobalt from Umicore andH.C. Starck fine grain Cr₃C₂, ZrC and VC.

TABLE 2a Chemical composition given in wt-%, 8 wt-% Co and rest = WC. CrV Zr N CW_Cr Me/Co 0.014 − (CW_Cr) * 0.008 2A 0.51 — 0.025 0.032 0.760.0020 0.0079 Inv 2B 0.51 0.0266 — 0.032 0.78 0.0038 0.0078 Inv 2C 0.51— — 0.032 0.69 NA NA Ref NA = not applicable

The powders were mixed by milling for 8 hours in a ball mill, thereafterpressed and sintered to bodies at 1410° C. for 1 hour in a protectiveatmosphere of Ar at a pressure of 40 mbar. After sintering the CW_Cr andHc were measured. CW_Cr and 0.014-(CW_Cr)*0.008 were determined and alsoreported in table 2a together with the Me/Co.

Furthermore the bodies were cut and crossections were ground andthereafter polished. Ten micrographs in suitable magnification for grainsize measurement were taken of each sample corresponding to the areagiven in table 2b. The grain size was measured using Jefferiesplanimetric method by the equivalent circle method.

TABLE 2b No. of grains Area μm2 grains/Area 2r (μm) 2A 5430 469.00 11.580.31 FIG. 2 2B 6070 732.80 8.28 0.36 2C 5980 1235.0 4.84 0.47 FIG. 1

Adding ppm-levels of Zr and V has a strong grain refining effect.

EXAMPLE 3

Cemented carbide mixtures 3A, 3B and 3C with the compositions accordingto table 3a were prepared. The WC-powder had an FSSS grain size of 0.9μm as supplied from H.C. Starck and fine grain (0.5 μm) cobalt fromUmicore and H.C. Starck fine grain Cr3C2, TiC and ZrC.

TABLE 3a Chemical composition given in wt-% Variant Co Cr Zr Ti % WC 3A10 0.43 — — Rest Reference 3B 10 1.0 0.012 0.014 Rest Invention 3C 100.6 0.06 — Rest Invention

The powders were mixed by milling for 8 hours in a ball mill, thereafterpressed and sintered to bodies at 1410° C. for 1 hour in a protectiveatmosphere of Ar at a pressure of 40 mbar. After sintering the CW_Cr andHc were measured. CW_Cr and 0.014-(CW_Cr)*0.008 were determined and arealso reported in table 3b together with the Me/Co.

TABLE 3b Variant CW_Cr Me/Co 0.014 − (CW_Cr) * 0.008 3A 0.90 — NA Ref 3B0.92 0.0025 0.0066 Invention 3C 0.90 0.0039 0.0068 Invention NA = notapplicable

Sintered bodies of each variant were cut in half, ground and polishedand thereafter etched with Murakami's regent for 2 minutes. An area ofcorresponding to 1 cm² of each variant was examined by light opticalmicroscopy and grains with a maximum length in any direction larger thanor equal to 5 μm were counted. The result is given in table 3c:

TABLE 3c Number of grains with max. length >= 5 μm Variant No of grains3A 76 Reference 3B 5 Invention 3C 17 Invention

The Zr and Zr+Ti ppm-levels reduces the amount of abnormal grains bymore than 50%.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. WC-Co cemented carbide comprising a composition of from about 3 toabout 15 wt-% Co and Cr such that the Cr/Co-ratio by weight is fromabout 0.05 to about 0.15, from about 0.01 to about 0.10 wt-% N, andbalance WC and in addition ppm levels of: one of the followingadditional elements V, Nb, Zr, Ta then with a grain size of WC of fromabout 0.1 to about 2.0 μm or mixtures of at least two of Ti, V, Nb, Zr,Ta, then with a grain size of the WC of more than about 0.3 up to about2.0 μm, the ratio ofMe/Co=(at-% Ti+at-% V+at-% Nb+at-% Zr+at-% Ta)/at-% Co is lower than orequal to about 0.014-(CW_Cr)*0.008 and higher than about 0.0005, with aCW_Cr ratio of from about 0.79 to about 0.95, withCW_Cr=(magnetic-% Co+1.13*wt % Cr)/wt % Co where magnetic-% Co is theweight percentage of magnetic Co and wt-% Co is the weight percentage ofCo in the cemented carbide.
 2. Cemented carbide of claim 1 wherein oneof the said additional elements is Zr.
 3. Cemented carbide of claim 1wherein said additional elements are Zr and Ti.
 4. Cemented carbide ofclaim 1 wherein said additional elements are Ti and Ta.
 5. Cementedcarbide of claim 1 wherein said additional elements are Ti and Ta with aN-content of more than about 0.02 wt-%.
 6. Cemented carbide of claim 1wherein the ratio Me/Co is higher than about 0.0007.
 7. Cemented carbideof claim 1 wherein the CW_Cr is from about 0.80 to about 0.92.
 8. Methodof making a WC-Co cemented carbide according to claim 1 comprisingmixing and wetmilling of powders of WC and Co, pressing and sinteringwherein ppm levels of one of the following additional elements V, Nb,Zr, Ta or mixtures of at least two of Ti, V, Nb, Zr, Ta, as pure metalsor as a carbides, nitrides and/or carbonitrides or mixtures thereof areadded to the powder mixture.